Nowadays' production of solar cells mainly relies on silicon. Crystalline silicon solar cells (including multi-crystalline and single-crystal) account for 90% of solar cell production [http://www.epia.org]. But a major issue is currently threatening the impressive 40% annual growth of both market and production: the world of photovoltaics is currently undergoing a deep silicon feedstock shortage [Photon International]. In addition, the price of a commercially available silicon solar module is driven by the cost of the semiconductor material which represents by itself almost half of the production cost of the finished module. Furthermore, independently of the material price, with currently industrially achievable surface recombination velocities (˜200 cm·s−1), the optimum thickness for achieving the highest efficiency on a crystalline silicon solar cell lies in the range of 50 μm, a lot thinner than the currently available 250 μm-thick wafers. Moreover below a few tens of microns, silicon becomes flexible. Encapsulated in a suitable and flexible material, it can provide a building block for new applications requiring lightweight, flexible, high efficiency solar modules, such as energy generating clothes, ambient intelligence devices, solar vehicles, etc. These arguments favor, independently of one another, the development of cells and/or wafers as thin as possible.
At the moment, silicon wafering (e.g. for solar cells) is still performed by wire saw techniques, which bring an important kerf loss of around 200 μm per wafer. The relative loss is thus even increasing for thinner substrates.
Silicon ribbon technology (W. Koch et al., Proceedings Td World PVSEC. 1998, p. 1254, Photon International, June 2004, R Janoch, R Wallace, J I Hanoka—CONFERENCE RECORD IEEE PHOTOVOLTAIC SPECIALISTS CONFERENCE, 1997), leads to the production of 150-μm-thick wafers. It is difficult to achieve thinner wafers with the conventional ribbon technologies. Moreover, the throughput of most ribbon technologies is low.
The Smart Cut technique relies on H+ implantation at a given depth in a wafer in order to weaken an underlying layer and detaching the top layer from its parent wafer. This technique is not cost competitive for solar applications [L. Shao et al., Applied Physics Letters 87, 091902, 2005, and S Bengtsson—Solid-State and Integrated Circuit Technology, 1998].
Other technologies (‘lift-off’ or ‘layer transfer’ approaches) rely on the creation of a weaker layer (crystal mismatch, porous layer, defect-full layer), on which a crystalline silicon layer is grown. The grown layer is then detached from the parent layer. [R. Brendel, Jpn. J. Appl. Phys. Vol. 40, Part 1, No. 7, 4431 (2001); C. S. Solanki, et al. (IMEC), Phys. Stat. Sol. (a), 182 (2000) 97; K. J. Weber et al., Appl. Phys. A, A69 (1999) 195].
Another alternative takes benefit of the fact that the kerf loss is reduced when a laser is used to cut the wafers. A full-laser wafering technique has not been performed yet but only thin silicon slices (1-2 mm large) have been produced, which are not ideal for solar cells because of the complexity of the assembling of the solar module. [A. W. Blakers et al., “Recent development in Sliver Cell Technology”, 20th European Photovoltaic Solar Energy Conference, 6-10 Jun. 2005, Barcelona, Spain].